— the trimmed H1 line text (without the leading #).
Optionalsubfolder: string— the brief's containing subfolder relative to analysis/daily/<date>/ (e.g. propositions or election-cycle/current). When provided, Rule E fires if the H1 matches the prettified label of either the full path or its leaf.
Validate a single H1 string against the five headline-quality rules. Pure function — exported for unit testing without filesystem I/O.